Let me know how you like my library. I'm thinking of adding some more
ergonomics to it:
1) support right folds so a for/list-like comprehension has better allocation
behavior.
2) add a keyword for-clause for starting an inner iteration so you don't have
to do this:
(for ([e0 (in-list l0)]
[e1 (in-list l1)])
(for ([e2 (in-list l2)]
[e3 (in-list l3)])
...body...))
but instead you can do this:
(for ([e0 (in-list l0)]
[e1 (in-list l1)]
#:start-inner-loop ;; suggest a better name please
[e2 (in-list l2)]
[e3 (in-list l3)])
...body...))
I seem to remember that for* is not equivalent to successive nesting of for
forms, but I can't remember how I reached this conclusion.
-Ian
----- Original Message -----
From: "Tim Nelson" <[email protected]>
To: "Pierpaolo Bernardi" <[email protected]>
Cc: "users" <[email protected]>
Sent: Monday, March 25, 2013 9:16:44 AM GMT -05:00 US/Canada Eastern
Subject: Re: [racket] for/hash and for/list: adding nothing, adding more than
one thing
Thanks everyone for the useful feedback. I'll check out all three options. :-)
On Fri, Mar 22, 2013 at 12:23 PM, Pierpaolo Bernardi < [email protected] >
wrote:
On Thu, Mar 21, 2013 at 11:55 PM, Tim Nelson < [email protected] > wrote:
> Dear All,
>
> I've recently started hacking Racket again, and am loving the for/*
> constructs.
> Awesome stuff. However, I have two questions about for/hash.
>
> (1) How can I *not* add a hash entry for a given iteration?
>
> When I use for/list, can I abuse the nature of empty and append* (I cringe
> as I write this):
>
> (append* (for/list ([i '(1 2 3 4 5)])
> (cond [(even? i) (list (* i 100))]
> [else empty])))
>
> Surely there is a better way to do this for for/list, and a way in general
> for for/hash?
>
> (2) How can I add multiple entries (that is, >1 key-value pair) in a given
> iteration?
>
> Again, using for/list I can cheat via append*:
>
> (append* (for/list ([i '(1 2 3 4 5)])
> (list (* i 100) (* i 200))))
>
> What's the right way to do this in for/hash?
To both of the questions, the answer is for/fold!
Cheers
P.
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